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<h3><a name="SECTION0025236000000000000000"></a><a name="doctest-warnings"></a>
<br>
23.2.3.6 Warnings
</h3>

<p>
<tt class="module"><a href="module-doctest.html">doctest</a></tt> is serious about requiring exact matches in expected
output.  If even a single character doesn't match, the test fails.  This
will probably surprise you a few times, as you learn exactly what Python
does and doesn't guarantee about output.  For example, when printing a
dict, Python doesn't guarantee that the key-value pairs will be printed
in any particular order, so a test like

<p>
<div class="verbatim"><pre>
&gt;&gt;&gt; foo()
{"Hermione": "hippogryph", "Harry": "broomstick"}
</pre></div>

<p>
is vulnerable!  One workaround is to do

<p>
<div class="verbatim"><pre>
&gt;&gt;&gt; foo() == {"Hermione": "hippogryph", "Harry": "broomstick"}
True
</pre></div>

<p>
instead.  Another is to do

<p>
<div class="verbatim"><pre>
&gt;&gt;&gt; d = foo().items()
&gt;&gt;&gt; d.sort()
&gt;&gt;&gt; d
[('Harry', 'broomstick'), ('Hermione', 'hippogryph')]
</pre></div>

<p>
There are others, but you get the idea.

<p>
Another bad idea is to print things that embed an object address, like

<p>
<div class="verbatim"><pre>
&gt;&gt;&gt; id(1.0) # certain to fail some of the time
7948648
&gt;&gt;&gt; class C: pass
&gt;&gt;&gt; C()   # the default repr() for instances embeds an address
&lt;__main__.C instance at 0x00AC18F0&gt;
</pre></div>

<p>
The <tt class="constant">ELLIPSIS</tt> directive gives a nice approach for the last
example:

<p>
<div class="verbatim"><pre>
&gt;&gt;&gt; C() #doctest: +ELLIPSIS
&lt;__main__.C instance at 0x...&gt;
</pre></div>

<p>
Floating-point numbers are also subject to small output variations across
platforms, because Python defers to the platform C library for float
formatting, and C libraries vary widely in quality here.

<p>
<div class="verbatim"><pre>
&gt;&gt;&gt; 1./7  # risky
0.14285714285714285
&gt;&gt;&gt; print 1./7 # safer
0.142857142857
&gt;&gt;&gt; print round(1./7, 6) # much safer
0.142857
</pre></div>

<p>
Numbers of the form <code>I/2.**J</code> are safe across all platforms, and I
often contrive doctest examples to produce numbers of that form:

<p>
<div class="verbatim"><pre>
&gt;&gt;&gt; 3./4  # utterly safe
0.75
</pre></div>

<p>
Simple fractions are also easier for people to understand, and that makes
for better documentation.

<p>

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